Hydraulic fluid composition

ABSTRACT

The present invention provides a hydraulic oil composition comprising: a lubricating base oil; and 0.2 to 40% by mass, based on a total amount of the hydraulic oil composition, of at least one copolymer selected from olefin copolymers having a number-average molecular weight of 18000 or lower and copolymers of an α-olefin and a dicarboxylic ester, having a number-average molecular weight of 20000 or lower.

TECHNICAL FIELD

The present invention relates to a hydraulic oil composition. Thepresent invention relates particularly to a hydraulic oil compositioncontaining a viscosity index improver and having a high energyefficiency.

BACKGROUND ART

In recent years, energy-saving hydraulic oils have been developed as oneof responses to global warming. There are some conventionalenergy-saving hydraulic oils allowing achieving the reduction of energyconsumption of apparatuses at starting, for example, by decreasing theirlow-temperature viscosity.

There are also developed energy-saving hydraulic oils whose viscositychange is made small by blending a viscosity index improver to therebyreduce energy consumption in the steady-state operation after the fluidtemperature is raised. In the energy-saving hydraulic oils, the fluidleakage (internal leakage) from construction machines' characteristicvarious hydraulic apparatus interiors is prevented by making small theviscosity change (making the viscosity index high) of the hydraulicoils, and the reduction of the energy consumption is achieved (forexample, see Patent Literatures 1 to 3).

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Laid-Open No.2005-307197

Patent Literature 2: Japanese Patent Application Laid-Open No.2011-046900

Patent Literature 3: Japanese Patent Application Laid-Open No.2012-180535

SUMMARY OF INVENTION Technical Problem

In the case of the energy-saving hydraulic oils as described in theabove Patent Literatures 1 to 3, however, the high viscosity index ofthe hydraulic oils causes an increase in the loss due to the plumbingresistance. Hence, even if the energy consumption can be reduced by theinternal leakage prevention, there is still room for improvement in thepoint of improving the energy efficiency of the hydraulic system as awhole.

The present invention has been achieved in consideration of such a realsituation, and an object thereof is to provide a hydraulic oilcomposition enabling both the internal leakage prevention and theplumbing resistance reduction to be compatibly achieved, and enablingthe energy efficiency of a hydraulic system as a whole to be improved.

Solution to Problem

As a result of exhaustive studies, the present inventors have found thata hydraulic oil composition obtained by blending a lubricating base oilwith a specific amount of a specific copolymer has viscositycharacteristics excellent in compatibly achieving both the internalleakage prevention and the plumbing resistance reduction of a hydraulicsystem, and this finding has led to the completion of the presentinvention.

That is, the present invention provides a hydraulic oil compositioncomprising a lubricating base oil, and 0.2 to 40% by mass, based on thetotal amount of the hydraulic oil composition, of at least one copolymerselected from olefin copolymers having a number-average molecular weightof 20000 or lower and copolymers of an α-olefin and a dicarboxylicester, having a number-average molecular weight of 20000 or lower.

Further in the above hydraulic oil composition, it is preferable thatthe hydraulic oil composition has the viscosity index of 155 or higher,and the ratio (A/B) of (A) a kinematic viscosity (unit: mm²/s) at 80° C.to (B) a shear viscosity (unit: mPa·s, shear condition: 10⁶/s) at 80° C.is 1.3 or lower.

Advantageous Effects of Invention

The hydraulic oil composition according to the present invention has alow kinematic viscosity as opposed to a high shear viscosity, enablesboth the internal leakage prevention and the plumbing resistancereduction to be compatibly achieved, and exhibits a remarkable effect ofenabling the energy efficiency of a hydraulic system as a whole to beimproved.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a preferred embodiment according to the present inventionwill be described.

A hydraulic oil composition according to an embodiment of the presentinvention comprises a lubricating base oil and at least one selectedfrom olefin copolymers having a number-average molecular weight of 20000or lower and copolymers, of an α-olefin and a dicarboxylic ester, havinga number-average molecular weight of 20000 or lower.

The lubricating base oil to be used in the present embodiment includesmineral oils, synthetic hydrocarbon oils, synthetic oxygen-containingoils, and fats and oils. These lubricating base oils can be used singlyor in combinations of two or more.

The mineral oil is not especially limited, but examples thereof includeparaffinic mineral oils or naphthenic mineral oils refined by subjectinglubricating oil fractions obtained by atmospheric pressure distillationand reduced pressure distillation of crude oils to suitably combinedrefining treatments including solvent deasphalting, solvent extraction,hydrocracking, solvent dewaxing, catalytic dewaxing, hydrorefining,sulfuric acid cleaning and clay treatment.

Examples of the synthetic hydrocarbon oil include poly-α-olefins(polybutene, 1-octene oligomers, 1-decene oligomers and the like),alkylbenzenes and alkylnaphthalenes.

As the synthetic oxygen-containing oil, there are used, for example,esters such as monoesters of a monohydric alcohol and a monovalent fattyacid, and polyol esters of a polyhydric alcohol and a monovalent fattyacid; and polyoxyalkylene glycols.

As the fats and oils, there are used, for example, vegetable fats andoils such as palm oil, palm kernel oil, rapeseed oil, soybean oil, higholeic rapeseed oil and high oleic sunflower oil.

Among these, mineral oils and synthetic hydrocarbon oils are preferablyused and mineral oils are more preferably used.

The kinematic viscosity at 40° C. of the lubricating base oil is notespecially limited, but is preferably 15 mm²/s or higher, morepreferably 20 mm²/s or higher, still more preferably 25 mm²/s or higher,and most preferably 30 mm²/s or higher. Further the kinematic viscosityat 40° C. of the lubricating base oil is preferably 50 mm²/s or lower,more preferably 45 mm²/s or lower, still more preferably 40 mm²/s orlower, and most preferably 35 mm²/s or lower. When the kinematicviscosity at 40° C. of the lubricating base oil is 15 mm²/s or higher,the case is preferable in the point of evaporation; and when thekinematic viscosity at 40° C. of the lubricating base oil is 50 mm²/s orlower, the case is preferable because the plumbing resistance can bereduced.

The viscosity index of the lubricating base oil is not especiallylimited, but is preferably 150 or higher, more preferably 160 or higher,still more preferably 170 or higher, and most preferably 175 or higher.When the viscosity index is 150 or higher, since the kinematic viscosityat low temperatures is suppressed in becoming high when the kinematicviscosity at high temperatures is secured, the case is preferable in thepoint of being capable of suppressing the efficiency decrease of ahydraulic system. On the other hand, the upper limit value of theviscosity index is not especially limited, but is, for example, 250.

Here, the “kinematic viscosity” and the “viscosity index” in the presentspecification mean values measured according to JIS K 2283.

The content of the lubricating base oil is preferably 50% by mass orhigher, more preferably 60% by mass or higher, and still more preferably70% by mass or higher based on the total amount of the hydraulic oilcomposition. Further the content of the lubricating base oil ispreferably 99% by mass or lower, and more preferably 98% by mass orlower based on the total amount of the hydraulic oil composition. Whenthe content of the lubricating base oil is 50% by mass or higher, theexcellent advantages of the hydraulic oil are easily fully exhibited.

The hydraulic oil composition according to the present embodimentcomprises at least one selected from olefin copolymers having anumber-average molecular weight of 20000 or lower and copolymers, of anα-olefin and a dicarboxylic ester, having a number-average molecularweight of 20000 or lower.

Here, the “number-average molecular weight” in the present specificationrefers to a number-average molecular weight in terms of polystyrenedetermined by gel permeation chromatography (GPC) (reference material:polystyrene).

The olefin copolymer is a cooligomer or a copolymer of ethylene and anα-olefin. The α-olefin includes propylene, 1-butene and 1-pentene, andpropylene is preferably used. The copolymer of ethylene and an α-olefinis not especially limited, and may be a random polymer or a blockpolymer.

The number-average molecular weight of the olefin copolymer is 18000 orlower, preferably 16000 or lower, more preferably 14000 or lower, andstill more preferably 10000 or lower. Further the number-averagemolecular weight of the olefin copolymer is preferably 700 or higher,more preferably 1000 or higher, and still more preferably 1500 orhigher. When the number-average molecular weight is 18000 or lower, thecase is preferable in the point of the pump efficiency; and when thenumber-average molecular weight is 700 or higher, the case is preferablebecause the effect of improving the viscosity index becomes large.

An example of the copolymer of an α-olefin and a dicarboxylic esterincludes a compound represented by the following formula (1).

In the above formula (1), R¹ represents a linear or branched alkylgroup; R² to R⁵ may be identical or different, and each representhydrogen, a linear or branched alkyl group, or an ester grouprepresented by —R⁶—CO₂R⁷ or —CO₂R⁸ (R⁶ represents a linear or branchedalkylene group; and R⁷ and R⁸ may be identical or different, and eachrepresent a linear or branched alkyl group); any two of R² to R⁵ are theabove ester group; and X and Y may be identical or different, and eachrepresent a positive number.

Here, a partial structure represented by the following formula (2) inthe above formula (1) is originated from the α-olefin, and as theα-olefin, one having 3 to 20 carbon atoms is used and one having 6 to 18carbon atoms is preferably used.

The α-olefin specifically includes propylene, 1-butene, 1-pentene,1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 1-undecene,1-dodecene, 1-tridecene, 1-tetradecene, 1-pentadecene, 1-hexadecene,1-heptadecene, 1-octadecene, 1-nonadecene and 1-eicosene.

Further a partial structure represented by the following formula (3) inthe above formula (1) is originated from the dicarboxylic ester.

The dicarboxylic ester specifically includes maleic acid, fumaric acid,citraconic acid, mesaconic acid and itaconic acid.

The number-average molecular weight of the copolymer of an α-olefin anda dicarboxylic ester is 20000 or lower, preferably 18000 or lower, morepreferably 14000 or lower, still more preferably 12000 or lower, andmost preferably 10000 or lower. When the number-average molecular weightis 20000 or lower, the case is preferable in the point of the pumpefficiency-improving capability. Further the number-average molecularweight of the copolymer of an α-olefin and a dicarboxylic ester is notespecially limited, but is preferably 5000 or higher, more preferably6000 or higher, and still more preferably 8000 or higher. When thenumber-average molecular weight is 5000 or higher, the case ispreferable in the point of the viscosity-improving capability.

The kinematic viscosity at 100° C. of the copolymer of an α-olefin and adicarboxylic ester is not especially limited, but is preferably 1 mm²/sor higher, more preferably 10 mm²/s or higher, still more preferably 50mm²/s or higher, and most preferably 200 mm²/s or higher; and furtherthat is preferably 5000 mm²/s or lower, more preferably 3000 mm²/s orlower, still more preferably 2000 mm²/s or lower, and most preferably1000 mm²/s or lower. When the kinematic viscosity at 100° C. is 1 mm²/sor higher, the case is preferable in the point of making a highviscosity; and when that is 5000 mm²/s or lower, the case is preferablein the point of handling in the production.

In the present embodiment, in the case of concurrently using the olefincopolymer and the copolymer of an α-olefin and a dicarboxylic ester,their ratio is not especially limited, and is arbitrary.

The content of the copolymers is 0.2 to 40% by mass based on the totalamount of the hydraulic oil composition. When the content of thecopolymers is 0.2% by mass or higher, the blending effect is easilyattained. Further when the content is 40% by mass or lower, the case ispreferable in the point of the solubility and the stability.

In the case of singly using the olefin copolymer as the copolymer, thecontent of the copolymer is preferably 20% by mass or lower, morepreferably 15% by mass or lower, still more preferably 10% by mass orlower, and most preferably 8% by mass or lower based on the total amountof the hydraulic oil composition. Further the content of the copolymeris preferably 0.2% by mass or higher, more preferably 0.5% by mass orhigher, still more preferably 1% by mass or higher, and most preferably3% by mass or higher based on the total amount of the hydraulic oilcomposition.

In the case of singly using the copolymer of an α-olefin and adicarboxylic ester as the copolymer, the content of the copolymer is 40%by mass or lower, preferably 35% by mass or lower, more preferably 30%by mass or lower, and still more preferably 25% by mass or lower basedon the total amount of the hydraulic oil composition. Further thecontent of the copolymer is preferably 1% by mass or higher, morepreferably 3% by mass or higher, still more preferably 5% by mass orhigher, and most preferably 10% by mass or higher based on the totalamount of the hydraulic oil composition.

In the case of concurrently using the olefin copolymer and the copolymerof an α-olefin and a dicarboxylic ester as the copolymers, the contentof the copolymers is 40% by mass or lower, preferably 35% by mass orlower, more preferably 30% by mass or lower, and still more preferably25% by mass or lower based on the total amount of the hydraulic oilcomposition. Further the content of the copolymers is 0.1% by mass orhigher, preferably 1% by mass or higher, more preferably 3% by mass orhigher, and still more preferably 5% by mass or higher based on thetotal amount of the hydraulic oil composition.

When the content of the olefin copolymer and/or the copolymer of anα-olefin and a dicarboxylic ester is the above given amount or larger,the blending effect is easily attained; and when that is the above givenamount or smaller, the case is preferable in the point of the solubilityand the stability.

The kinematic viscosity at 40° C. of the hydraulic oil composition isnot especially limited, but is preferably 20 mm²/s or higher, morepreferably 30 mm²/s or higher, still more preferably 40 mm²/s or higher,and most preferably 45 mm²/s or higher. Further the kinematic viscosityat 40° C. of the hydraulic oil composition is preferably 80 mm²/s orlower, more preferably 70 mm²/s or lower, still more preferably 60 mm²/sor lower, and most preferably 50 mm²/s or lower. When the kinematicviscosity at 40° C. of the hydraulic oil composition is 20 mm²/s orhigher, the case is preferable in the point of the durability of ahydraulic system; and when that is 80 mm²/s or lower, the case ispreferable in the point of the friction reduction.

The viscosity index of the hydraulic oil composition is preferably 150or higher, more preferably 155 or higher, still more preferably 160 orhigher, and most preferably 165 or higher. When the viscosity index is150 or higher, the case is preferable because an optimum viscosity rangecan be held over a broad temperature range. On the other hand, the upperlimit value of the viscosity index is not especially limited, but is,for example, 250.

The ratio (A/B) of (A) a kinematic viscosity at 80° C. to a shearviscosity (unit: mPa·s, shear condition: 10⁶/s) at 80° C. with respectto the hydraulic oil composition is not especially limited, but ispreferably 1.4 or lower, more preferably 1.3 or lower, still morepreferably 1.25 or lower, and most preferably 1.2 or lower. When theabove A/B is 1.4 or lower, the case is preferable in the point of thepump efficiency and the plumbing resistance. On the other hand, thelower limit value of the above A/B is not especially limited, but is,for example, 1.1.

Here, the “shear viscosity” in the present specification means a valuemeasured according to ASTM (D4741, D4683, D6616), CEC (L-36A-90).

The hydraulic oil composition according to the present embodiment, inorder to more improve its excellent advantages, can further comprise, asrequired, an extreme pressure agent, an antioxidant, a pour pointdepressant, a rust-preventive agent, a metal deactivator, a viscosityindex improver, an antifoaming agent, a demulsifier, an oiliness agentand the like. These additives may be used singly or in combinations oftwo or more.

The extreme pressure agent includes sulfur compounds such as estersulfides, sulfurized fats and oils and polysulfides, zincdithiophosphate, and phosphorus compounds, and it is preferable thatphosphorus compounds are used. The phosphorus compounds specificallyinclude phosphate esters, acidic phosphate esters, amine salts of acidicphosphate esters, chlorinated phosphate esters, phosphite esters andphosphorothionate. The phosphorus compounds include esters of phosphoricacid, phosphorous acid or thiophosphoric acid with an alkanol or apolyetheric alcohol, and their derivatives.

Among the above phosphorus compounds, since higher antiwear property canbe provided, phosphate esters, acidic phosphate esters, amine salts ofacidic phosphate esters are preferable, and among these, phosphateesters are more preferable. It is preferable that the content of theextreme pressure agent is 0.05 to 5% by mass based on the total amountof the hydraulic oil composition.

Examples of the antioxidant include phenolic compounds such as2,6-ditertiary-butyl-p-cresol (DBPC), aromatic amines such asphenyl-α-naphthylamine, hindered amine compounds, phosphite esters andorganometal compounds. It is preferable that the content of the phenolicantioxidant is 0.01 to 2% by mass based on the total amount of thehydraulic oil composition. Further it is preferable that the content ofthe amine-based antioxidant is 0.001 to 2% by mass based on the totalamount of the hydraulic oil composition.

Examples of the pour point depressant are copolymers of at least onemonomer selected from acrylate esters and methacrylate esters, andhydrogenated substances thereof. It is preferable that the content ofthe pour point depressant is 0.01 to 5% by mass based on the totalamount of the hydraulic oil composition.

Examples of the rust-preventive agent are amino acid derivatives,partial esters of polyhydric alcohols; esters such as lanolin fatty acidesters, alkyl succinate esters and alkenyl succinate esters; sarcosine;polyhydric alcohol partial esters such as sorbitan fatty acid esters;metal soaps such as fatty acid metal salts, lanolin fatty acid metalsalts and oxidized wax metal salts; sulfonates such as calcium sulfonateand barium sulfonate; oxidized waxes; amines; phosphoric acid; andphosphate salts. It is preferable that the content of therust-preventive agent is 0.01 to 5% by mass based on the total amount ofthe hydraulic oil composition.

Examples of the metal deactivator are benzotriazole compounds,thiadiazole compounds and imidazole compounds. It is preferable that thecontent of the metal deactivator is 0.001 to 1% by mass based on thetotal amount of the hydraulic oil composition.

The hydraulic oil composition according to the present embodiment canfurther comprise a viscosity index improver other than the abovecopolymers. Specific examples thereof include non-dispersive viscosityindex improvers such as copolymers of at least one monomer selected frommethacrylate esters and hydrogenated substances thereof,polyisobutylenes and hydrogenated substances thereof, hydrogenatedstyrene-diene copolymers and polyalkylstyrenes. It is preferable thatthe content of the viscosity index improver other than the abovecopolymers is 0.01 to 15% by mass based on the total amount of thehydraulic oil composition.

Examples of the antifoaming agent are silicones such asdimethylsilicones and fiuorosilicones. It is preferable that the contentof the antifoaming agent is 0.001 to 0.05% by mass based on the totalamount of the hydraulic oil composition.

Examples of the demulsifier include polyoxyalkylene glycols,polyoxyalkylene alkyl ethers, polyoxyalkylene alkylamides andpolyoxyalkylene fatty acid esters.

The oiliness agent includes fatty acids, esters and alcohols. It ispreferable that the content of the oiliness agent is 0.01 to 0.5% bymass based on the total amount of the hydraulic oil composition.

EXAMPLES

Hereinafter, the present invention will be described more specificallyby way of Examples and Comparative Examples, but the present inventionis not any more limited to these contents.

In Examples 1 to 4 and Comparative Examples 1 to 4, hydraulic oilcompositions were each prepared by blending a lubricating base oil andadditives in a composition shown in Table 1 and Table 2. The lubricatingbase oils and the additives used in the Examples and the ComparativeExamples are as follows.

<Lubricating Base Oils>

Base oil 1: hydrorefined mineral oil (total aromatic content: 0.0% bymass, sulfur content: 10 ppm by mass or lower, 40° C. kinematicviscosity: 20 mm²/s, viscosity index: 124)

Base oil 2: hydrorefined mineral oil (total aromatic content: 0.0% bymass, sulfur content: 10 ppm by mass or lower, 40° C. kinematicviscosity: 26 mm²/sec, viscosity index: 131)

Base oil 3: hydrorefined mineral oil (total aromatic content: 0.0% bymass, sulfur content: 10 ppm by mass or lower, 40° C. kinematicviscosity: 46 mm²/sec, viscosity index: 127)

Here, the total aromatic content was measured according tosilica-alumina gel chromatography described in “Separation ofHigh-Boiling Petroleum Distillates Using Gradient Elution ThroughDual-Packed (Silica Gel-Alumina Gel) Adsorption Columns,” AnalyticalChemistry, Vol. 44, No. 6, (1972), pp. 915-919.

Further the sulfur content was measured according to ASTM D4951,“Standard Test Method for Determination of Additive Elements inLubricating Oils by Inductively Coupled Plasma Atomic EmissionSpectrometry.”

Further the kinematic viscosity and the viscosity index were measuredaccording to JIS K 2283.

<Viscosity Index Improvers>

A: ethylene-propylene copolymer (Mitsui Chemicals, Inc., Lucant HC2000,number-average molecular weight: 13100)

B: copolymer of an α-olefin and a dicarboxylic ester (Ketjenlube,KL2700, number-average molecular weight: 9800, kinematic viscosity at100° C.: 700 mm²/sec)

C: styrene-diene copolymer (Infineum International Ltd., SV151,number-average molecular weight: 144000)

D: polymethacrylate (Sanyo Chemical Industries, Ltd., number-averagemolecular weight: 40000)

E: polymethacrylate (Sanyo Chemical Industries, Ltd., number-averagemolecular weight: 100000)

F: olefin copolymer (Chevron Corp., Paratone 8451, number-averagemolecular weight: 230000)

<Other Additives>

In Examples 1 to 4 and Comparative Examples 1 to 4, as other additives,tricresyl phosphate, 2,6-ditertiary-butyl-p-cresol (DBPC) and a pourpoint depressant were each blended in 0.5% by mass based on the totalamount of the hydraulic oil composition.

Each property was measured for each hydraulic oil composition obtainedin Examples 1 to 4 and Comparative Examples 1 to 4 as described below.The results are shown in Table 1 and Table 2.

The kinematic viscosity and the viscosity index: which were measuredaccording to HS K 2283.

The shear viscosity: which was measured according to ASTM (D4741, D4683,D6616), CRC (L-36A-90), at 80° C. at a shear condition of 10⁶/s. Ameasuring instrument used was a USV (Ultra Shear Viscometer) viscometer,manufactured by PCS Instruments.

[HPV35+35 Pump Test]

An HPV35+35 pump test was carried out on each hydraulic oil compositionobtained in Examples 1 to 4 and Comparative Examples 1 to 4.Specifically, the rotational torque of the pump was measured under thefollowing test condition, and the total efficiency was calculated. Theresults are shown in Table 1 and Table 1

The pump name: Komatsu HPV35+35

The discharge volume+the drain volume: 40 L/min

The pump type: a swash plate type

The oil temperature: 80° C.

The pressure: no load, 35 MPa

The rotation of the pump: 2100 rpm

TABLE 1 Example 1 Example 2 Example 3 Example 4 Composition base oil 1balance — balance balance (% by mass) base oil 2 — balance — — base oil3 — — — — viscosity index improver A 7.5 5 — 2.5 viscosity indeximprover B — — 19 9.5 viscosity index improver C — — — — viscosity indeximprover D — — — — viscosity index improver E — — — — viscosity indeximprover F — — — — tricresyl phosphate 0.5 0.5 0.5 0.5 DBPC 0.5 0.5 0.50.5 pour point depressant 0.5 0.5 0.5 0.5 Properties kinematic viscosityat 40° C. (mm²/s) 46.34 46.59 46.48 44.64 kinematic viscosity at 80° C.(mm²/s) 13.59 13.21 13.12 12.71 kinematic viscosity at 100° C. (mm²/s)8.74 8.42 8.35 8.12 viscosity index 171 159 157 157 shear viscosity at80° C. (mPa · s) 11.43 11.19 11.34 11.13 kinematic viscosity at 80° C./1.19 1.18 1.16 1.14 shear viscosity at 80° C. Total Efficiency (%) of66.5 65.8 65.8 65.7 HPV35 + 35 Pump Test [35 MPa, 80° C.]

TABLE 2 Comp. Comp. Comp. Comp. Example 1 Example 2 Example 3 Example 4Composition base oil 1 — — — — (% by mass) base oil 2 — balance balancebalance base oil 3 balance — — — viscosity index improver A — — — —viscosity index improver B — — — — viscosity index improver C — 11.5 — —viscosity index improver D — 5 — — viscosity index improver E — — 10 —viscosity index improver F — — — 7 tricresyl phosphate 0.5 0.5 0.5 0.5DBPC 0.5 0.5 0.5 0.5 pour point depressant 0.5 0.5 0.5 0.5 Propertieskinematic viscosity at 40° C. (mm²/s) 45.21 44.97 47.30 45.23 kinematicviscosity at 80° C. (mm²/s) 11.98 13.75 15.70 13.08 kinematic viscosityat 100° C. (mm²/s) 7.51 8.94 10.44 8.38 viscosity index 132 184 218 164shear viscosity at 80° C. (mPa · s) 10.10 9.12 11.19 8.70 kinematicviscosity at 80° C./ 1.19 1.51 1.40 1.50 shear viscosity at 80° C. TotalEfficiency (%) of 64.7 64.0 65.1 63.6 HPV35 + 35 Pump Test [35 MPa, 80°C.]

1. A hydraulic oil composition comprising: a lubricating base oil; and0.2 to 40% by mass, based on a total amount of the hydraulic oilcomposition, of at least one copolymer selected from olefin copolymershaving a number-average molecular weight of 18000 or lower andcopolymers of an α-olefin and a dicarboxylic ester, having anumber-average molecular weight of 20000 or lower.
 2. The hydraulic oilcomposition according to claim 1, wherein the hydraulic oil compositionhas a viscosity index of 150 or higher; and a ratio (A/B) of (A) akinematic viscosity (unit: mm²/s) at 80° C. to (B) a shear viscosity(unit: mPa·s, shear condition: 10⁶/s) at 80° C. is 1.3 or lower.